The present invention relates to a co-packaged field effect transistor (FET) and integrated circuit (IC) for a portable electronic device and, more particularly, to a co-packaged FET and IC such as for a switching power supply employing at least one FET, a Schottky diode, and an appropriate control circuit IC.
With reference to FIG. 1, it is known to use FET transistors in a forward switching power supply which, for example, is a buck power supply 10. The power supply 10 is coupled to a source of input DC power, Vin, having positive and negative terminals. An input filter capacitor 12 is coupled across the positive and negative terminals of Vin to provide a source of local energy storage.
First and second switching transistors Q1, Q2 are coupled in series across the positive and negative terminals of Vin. It is known that transistors Q1 and Q2 may be MOS gated FETs, each having a respective gate, drain, and source terminal. As is known in the art, a control IC 16 supplies gating signals to the gate terminals G1, G2 of transistors Q1 and Q2, respectively, to produce a pulse with modulated (PWM) signal at the junctions of transistors Q1, Q2.
A Schottky diode D1 is coupled in an anti-parallel configuration with transistor Q2 in order to provide a free-wheeling current path in a shunt across the transistor, as is known in the art.
In order to provide a relatively steady and smooth supply of DC output power at Vo, a second order filter is provided which utilizes a series coupled inductor L and a shunt coupled capacitor 14, as is also known in the art. The second order filter attenuates substantially all of the high frequency components of the PWM signal to produce the DC output voltage across the positive and negative terminals of Vo.
A sensing resistor Rs is provided in series with the inductor L so that a voltage may be developed across the resistor which is indicative of the current flowing through the inductor L. The sensed voltage is input to the control IC 16 so that the switching transistors Q1, Q2 may be properly controlled, for example by current mode control. Further, a resistor divider is coupled from the positive terminal to the negative terminal of Vo and employs resistors R1 and R2. A sensed voltage is taken from the junction of resistors R1 and R2 and is input to the control IC 16. The sensed voltage provides information as to the voltage level at Vo and is also used by the control IC 16 to properly bias transistors Q1 and Q2, for example by voltage mode control.
Because the switching frequency of transistors Q1 and Q2 may be relatively high and the current levels flowing through the transistors may also be relatively high, packaging considerations become critical. For example, transistors Q1 and Q2, as well as diode D1, may operate at elevated temperatures due to the high switching frequencies and high current conditions. Further, the lengths of the interconnections between the transistors Q1 and Q2, the control IC 16, and diode D1 may produce undesired stray inductances which cause excessive noise, excessive heat dissipation and spurious circuit operation.
Additionally, when the switching power supply 10 is incorporated into an electronic device which, for example, is a portable electronic device, the portion of the total circuit board area that is allocated to the power supply 10 becomes critical. Typically, power components such as transistors Q1, Q2 and diode D1 have relatively large footprints. TO-220 packages are often used to house transistors Q1 and Q2, which are relatively large and have a relatively low silicon to footprint ratio (about 15%).
Consequently, the designers of portable electronic devices are under ever increasing pressure to reduce the amount of area and/or volume utilized by the power supply 10.
Accordingly, a novel package for encapsulating power components and control circuitry to reduce the size and weight of the package, as well as to improve the circuit performance, by minimizing the parasitic inductances is needed in the art.